Propylene glycol is a valuable commodity chemical that has a broad range of uses such as for antifreeze. Propylene glycol is currently made from hydrocarbon feedstocks and by hydrogenolysis of glycerin.
Schreck, et al., in U.S. Pat. No. 9,399,610 B2, disclose improved, continuous processes for the conversion of carbohydrates to ethylene glycol and propylene glycol. They disclose using a reactor for the conversion of carbohydrates to the glycols which has a first zone comprising a retro-aldol catalyst and a second zone comprising a retro-aldol and reducing catalyst. Where the feed is an aldose, glycolaldehyde from the retro-aldol reaction is hydrogenated in the second zone of the reactor to ethylene glycol. They also disclose using ketose as the carbohydrate to produce propylene glycol.
Nevertheless, challenges still remain to further enhance the selectivity of the conversion of carbohydrates to glycols. These challenges are not insignificant due to the myriad of reactions that can occur under the conditions required for the retro-aldol reaction and for the hydrogenation, including, but not limited to, hydrogenation of sugars to hexitol or pentitol (herein referred to as alkitols) and the formation of side products such as methane, methanol, ethanol, propanol, glycerin, 1,2-butanediol, threitol, and humins. Although some side products may be marketable, their recovery to meet merchant grade specification can be costly.